1. Field of the Invention
This invention relates generally to switching and transmission techniques in telephone communication systems, and more particularly to an improved switching technique for connecting several pulse code modulated channels in a typical PCM system for conferencing. More particularly still, the invention relates to an improved switching and transmission technique for use in such a PCM system for reducing the common equipment required for a specified number of terminations, using a method and arrangement of channel allocation as compared to the usual dedicated channel arrangement.
2. Description of the Prior Art
In general, in most prior art systems wherein analog conference circuits are provided, the analog signals of all participants are compared and then all signals to an individual participant with the exception of the signals generated by that individual himself are extended. A variety of schemes such as direct multiplexing, hybrid repeaters, transistorized analog circuitry, etc. have been utilized. Various solutions to the problems of cross talk, echo loss, impedance matching, sidetone, etc. have been disclosed. Some of the techniques employed are shown in U.S. Pat. Nos. 3,116,369; 3,144,518; 3,170,042 and 3,301,962. None of the techniques disclosed employ the digital approach and hence are of little significance in the field of pulse code modulated telephony. A conference circuit for a time division multiplex telephone communication system is disclosed in U.S. Pat. No. 3,551,600. In this disclosure, digital techniques are employed. However, the time division signals are still in analog form and after being acted upon by the conference circuit, they must be reconverted from digital to analog form.
In U.S. Pat. No. 3,699,264, there is disclosed an algorithm for providing digital pulse code modulated conferences. In the disclosed system, during every time frame, an active speaker sample is sought. The active speaker is identified by comparing each participant's digital sample during the following time frame. The active speaker sample is transmitted to all participants in the conferencing arrangement. A last speaker sample is transmitted to the active speaker. With this approach, a 3-way conferencing per circuit is possible in its digital form.
While the arrangement disclosed in this U.S. Pat. No. 3,699,264 has found application in various systems, in existing commercial PABX systems having a large number of trunks, a very large number of 3-way conferencing circuits would be required. Accordingly, it would be desirable to provide in such systems, a 3-way conferencing facility through the switching network. As indicated above, one aspect of the present invention relates to such a switching technique for connecting several pulse code modulated channels for conferencing.
Presently known all-time pulse code modulated systems generally are arranged with the transmit side of each inlet appearing in a fixed position on one side of the switching network, and with the receive side of each inlet appearing in a fixed position on the other side thereof. The operation is such that all information entering from the input pulse code modulated (PCM) bus is stored and made available at the output under the control of a control memory which contains information regarding all network connections.
More particularly, in such systems, in a normal operation, each channel arriving on the input bus is written cyclically into a fixed location in an information memory. At this time, the control memory word corresponding to that channel is read for addressing the information memory. The content of the information memory at this address (which represents a PCM sample from the channel engaged in a conversation with the other channel) is read out. The same operation of writing the information memory, reading the control memory for the information memory address, and reading the information memory is performed during the other channels time slot. In any given channel location, the control memory contains the address of the channel to which it is to be connected. Such all-time PCM systems do not have the ability to provide a double connection for conferencing.